Oil sands are a type of unconventional petroleum deposit that contain naturally occurring mixtures of sand, clay, water, and a dense and extremely viscous form of petroleum technically referred to as “bitumen,” but which may also be called heavy oil or tar. Many countries have large deposits of oil sands, including the United States, Russia, and various countries in the Middle East, but the world's largest deposits occur in Canada and Venezuela.
Bitumen is a thick, sticky form of crude oil, so heavy and viscous that it will not flow unless heated or diluted with lighter hydrocarbons. At room temperature, bitumen is much like cold molasses. Often times, the viscosity can be in excess of 1,000,000 cP.
Due to their high viscosity, these heavy oils are hard to mobilize, and they generally must be made to flow in order to produce and transport them. One common way to heat bitumen is by injecting steam into the reservoir. The quality of the injected fluid is very important to transferring heat to the reservoir to allow bitumen to be mobilized. Quality in this case is defined as percentage of the injected fluid in the gas phase. The target fluid quality is near 100% vapor, however, injected fluid in parts of the well can have a quality below 50 percent (more than 50% liquid) due to heat loss along the wellbore.
Steam Assisted Gravity Drainage (SAGD) is the most extensively used technique for in situ recovery of bitumen resources in the McMurray Formation in the Alberta Oil Sands and other reservoirs containing viscous hydrocarbons. In a typical SAGD process, two horizontal wells are vertically spaced by 4 to 10 meters (m). The production well is located near the bottom of the pay and the steam injection well is located directly above and parallel to the production well. In SAGD, steam is injected continuously into the injection well, where it rises in the reservoir and forms a steam chamber.
With continuous steam injection, the steam chamber will continue to grow upward and laterally into the surrounding formation. At the interface between the steam chamber and cold oil, steam condenses and heat is transferred to the surrounding oil. This heated oil becomes mobile and drains, together with the condensed water from the steam, into the production well due to gravity segregation within the steam chamber.
This use of gravity gives SAGD an advantage over conventional steam injection methods. SAGD employs gravity as the driving force and the heated oil remains warm and movable when flowing toward the production well. In contrast, conventional steam injection displaces oil to a cold area where its viscosity increases and the oil mobility is again reduced.
Although quite successful, SAGD does require enormous amounts of water in order to generate a barrel of oil. Some estimates provide that 1 barrel of oil from the Athabasca oil sands requires on average 2 to 3 barrels of water, although with recycling the total amount can be reduced to 0.5 barrel. In addition to using a precious resource, additional costs are added to convert those barrels of water to high quality steam for downhole injection. Therefore, any technology that can reduce water or steam consumption has the potential to have significant positive environmental and cost impact.
One concept for reducing water consumption is the “multilateral” or “fishbone” well configuration idea. The concept of fishbone wells for non-thermal horizontal wells was developed by Petrozuata in Venezuela in 1999. That operation was a cold, viscous oil development in the Faja del Orinoco Heavy Oil Belt. The basic concept was to drill open-hole side lateral wells or “ribs” off the main spine of a producing well prior to running slotted liner into the spine of the well (FIG. 1).
A variety of multilateral well configurations are possible (see FIG. 2). Such ribs appear to significantly contribute to the productivity of the wells when compared to wells without the ribs in similar geology (FIG. 3).
The advantages of multilateral wells include:
1) Higher Production. In the cases where thin pools are targeted, vertical wells yield small contact with the reservoir, which causes lower production. Drilling several laterals in thin reservoirs and increasing contact improves recovery.
2) Decreased Water/Gas Coning. Coning is aggravated by pressure gradients that exceed the gravity forces that stabilize the fluid contacts (oil/water or gas/water). The position of the laterals within the producing formation provides enough distance to the water zone and to the gas zone to facilitate higher withdrawal rates and lower pressure gradients. Therefore, gas/water coning can be prevented or reduced.
3) Improved sweep efficiency. By using multilateral wells, the sweep efficiency may be improved and/or the recovery may be increased due to the area covered by the laterals.
4) Faster Recovery. The reservoir contact is higher in multilateral wells leading to increased production rates than that of single vertical or horizontal wells.
5) Decreased environmental impact. To the extent that the overall length of wells is reduced by sharing mother-bores, the volume of consumed drilling fluids and the generated cuttings during drilling multilateral wells can be reduced. Additionally, there may be a reduction of wellpad number. Therefore, the impact of the multilateral wells on the environment may be reduced.
6) Saving time and cost. Drilling several laterals in a single well will result in substantial time and cost saving in comparison with drilling several wells in the reservoir.
Lateral wells have been used for various methods in the patent literature. For example, EP2193251 discloses a method of drilling multiple short laterals that are of smaller diameter, and these multiple short laterals can be drilled at the same depth from the same main wellbore, so as to perform treatments in and from the small laterals to adapt or correct the performance of the main well, the formation properties, the formation fluids and the change of porosity and permeability of the formation. However, the short laterals do not address the issue where the prism or wedge between two adjacent SAGD well pairs is hard to produce/deplete.
US20110036576 discloses a method of injecting a treatment fluid through a lateral injection well such that the hydrocarbon can be treated by the treatment fluid before production. However, the addition of treatment fluid is known in the field and this well configuration does not increase the contact with the hydrocarbon reservoir.
Although a potential improvement, the multilateral well methods can have disadvantages too. One disadvantage is that fishbone wells are more complex to drill and clean up. Indeed, some estimate that multilaterals cost about 20% more to drill and complete than conventional slotted liner wells. Another disadvantage is increased risk of accident or damage, due to the complexity of the operations and tools. Sand control can also be difficult. In drilling multilateral wells, the mother well bore can be cased to control sand production, however, the legs branched off the mother well bore are typically open hole. Therefore, the sand control from the branches is not easy to perform. There is also increased difficulty in modeling and prediction due to the sophisticated architecture of multilateral wells.
Another area of uncertainty with the fishbone concept is whether the ribs will establish and maintain communication with the offset steam chambers, or will the open-hole ribs collapse early and block flow. One of the characteristics of the Athabasca Oil Sands is that they are unconsolidated sands that are bound by the million-plus centipoise bitumen. When heated to 50-80° C. the bitumen becomes slightly mobile. At this point the open-hole rib could collapse. If so, flow would slow to a trickle, temperature would drop, and the rib would be plugged. However, if the conduit remains open at least long enough that the bitumen in the near vicinity is swept away with the warm steam condensate before the sand grains collapse, then it may be possible that a very high permeability, high water saturation channel might remain even with the collapse of the rib. In this case, the desired conduit would still remain effective.
Another uncertainty with many ribs along a fishbone infill producer of this type is that one rib may tend to develop preferentially at the expense of all the other ribs leading to very poor conformance and poor results. This would imply that some form of inflow control may be warranted along the fishbone infill liner to encourage more uniform development of all the ribs.
Therefore, although beneficial, the multilateral well concept could be further developed to address some of these disadvantages or uncertainties.